1. Field of the Invention
The present invention relates to a thermosensitive recording medium.
2. Discussion of the Related Art
Thermosensitive recording media typically have a structure in which a thermosensitive coloring layer (i.e., a thermosensitive recording layer) which colors upon application of heat thereto is formed on a support such as paper, synthetic paper and resin films. Thermal printers having printing head such as thermal printheads are typically used for coloring the recording media (i.e., for recording images in the recording media).
Such a thermal recording method has the following advantages against the other recording methods:
(1) recording can be performed at a relatively high speed using a simple image forming apparatus because development and fixation processes are not needed in the image forming apparatus; and
(2) the costs of the recording media are relatively low.
Therefore, thermosensitive recording media have been used for various applications as follows:
(1) POS (point of sales) labels for foods, boxed lunches, dairy dishes, etc.;
(2) Copy papers;
(3) Receiving papers for facsimile machines; and
(4) Tickets and receipts.
Among these applications, the application for POS labels are remarkably expanding. When thermosensitive recording media are used for the labels for hot foods and dairy dishes, a problem which often occurs is that the information recorded in the labels cannot be recognized because of coloring of the background areas of the information printed in the labels.
International Publication No. WO99/51444 discloses a sensitizer such as bis(4-methylbenzyl) oxalate, bis(4-chlorobenzyl) oxalate, acetoacetic o-chloroanilide, diphenylsulfone, stearic acid amide, etc., to improve the thermosensitivity of the resultant thermosensitive recording medium. However, this recording material has drawbacks such that the color density is low, and the thermosensitivity is not satisfactory.
Japanese Laid-Open Patent Publication No. 61-246088 discloses that 4-acetylbiphenyl is useful as a sensitizer for a thermosensitive recording medium. However, even when such a sensitizer is used, the recorded images in the recording medium have unsatisfactory heat resistance. In addition, the background area of the images has also unsatisfactory heat resistance.
Because of these reasons, a need exists for a thermosensitive recording medium having a combination of high thermosensitivity and good heat resistance so as to be used for the applications such as labels for hot foods and dishes.
Accordingly, an object of the present invention is to provide a thermosensitive recording medium having a combination of high thermosensitivity and good heat resistance so as to be used for the applications such as labels for hot foods and dishes.
To achieve such objects, the present invention contemplates the provision of a thermosensitive recording medium including a thermosensitive recording layer formed overlying a support, where the recording layer includes at lease a leuco dye, a color developer and a sensitizer, wherein the color developer includes a compound selected from the group consisting of methylenebis(2-naphthol), compounds having a formula selected from the group consisting of the following formulae (1) to (3): 
and condensation products of a polyhydric alcohol component including a polyhydric alcohol having three or more hydroxyl groups with a carboxylic acid component including 4-hydroxybenzoic acid compound having the following formula (4): 
wherein m is 0 or an integer of from 1 to 2, and wherein the sensitizer includes 4-acetylbiphenyl.
The polyhydric alcohol having three or more hydroxyl groups preferably has the following formula (6): 
wherein n is 0 or an integer of from 1 to 9; R3 represents a hydroxymethyl group or an alkyl group having 1 to 8 carbon atoms; and each R4 independently represents a hydroxymethyl group or an alkyl group having 1 to 8 carbon atoms (i.e., for n greater than 1, such that there are two or more of R4 present, each R4 may be the same as or different from any other R4).
The leuco dye preferably includes 3-diethylamino-6-ethyl-7-(3-methylanilino)fluoran.
It is preferable that the thermosensitive layer preferably further includes a compound having the following formula (5): 
wherein X represents a hydrogen atom or a chlorine atom; and R1 and R2 independently represent a hydrogen atom or an alkyl group having from 1 to 8 carbon atoms except that R1 and R2 are not both a hydrogen atom.
The thermosensitive recording medium may include a protective layer on the thermosensitive recording layer, which preferably includes a compound having formula (5).
In addition, it is preferable that the recording layer further includes at least one of tetrakis(1,2,2,6,6-pentamethyl-4-piperidyl)-1,2,3,4-butanetetracarboxylate and tetrakis(2,2,6,6-tetramethyl-4-piperidyl)-1,2,3,4-butanetetracarboxylate.
Further, the thermosensitive recording medium may include an undercoat layer between the support and the thermosensitive recording layer, which preferably includes hollow particles having a hollow rate not less than 30% and a volume average particle diameter of from 0.4 to 10 xcexcm.